The present disclosure relates to a calculation apparatus and a processing apparatus. For example, the present disclosure relates to a calculation apparatus and a processing apparatus for controlling a motor.
In related art, a three-phase brushless motor is known as a three-phase motor. The three-phase brushless motor has high durability because it has no brush to be worn out, and thus it has been widely used. The most common three-phase brushless motor generates rotational torque by applying a three-phase alternating current to a three-phase coil. Square waves have been widely used as the three-phase current because it is easy to fabricate a driving circuit with them. However, in recent years, sinusoidal waves have been used to control sinusoidal wave current drive to thereby achieve improved quietness and driving efficiency. Furthermore, vector control that precisely controls amplitude and phase of a three-phase sinusoidal current is becoming common.
For the vector control, it is necessary to detect a phase current for at least two phases. For example, Japanese Unexamined Patent Application Publication No. 2006-353073 discloses an inverter apparatus. In this inverter apparatus, a shunt resistor is provided between lower arm switching elements of two phases among the lower arm switching elements of three phases and a negative side of the DC power supply. The same ON periods are deleted in all three phases from ON periods of upper arm switching elements within a carrier cycle, and the phase current for two phases in the phases where the shunt resistors are provided are detected. Then, the inverter apparatus can detect a phase current with a compact size and with high vibration resistance without requiring development of complicated control software and without generating current distortion.
However, it is costly to use two current sensors in order to detect the phase current for two phases. To solve this problem, there is a method in which one current sensor detects a bus current between an inverter and a DC power supply, and reproduces an original phase current (at least for two phases) from the detected current. This method is referred to as one shunt current detection method (single shunt current detection method). In this one shunt current detection method, by sampling an output signal of the current sensor at appropriate timings, it is possible to detect the phase current of the phase at which a voltage level becomes maximum (maximum phase) and the phase current of the phase at which the voltage becomes minimum (minimum phase), i.e., to detect a current for two phases.
In the one shunt current detection method, the bus current appears only when one or two phases of PWM (Pulse Width Modulation) inverters among three-phases of PWM inverters are on. Thus, in order to obtain current levels for two phases from the bus current, it is necessary to sample two bus current levels, which are a bus current level in the section where only one phase is on, and the bus current level in the section where two phases are on. In the one shunt current detection method, the number of current sensors is reduced to one, so that the cost of the sensor is low.
However, since the three-phase PWM pulses are PWM-modulated three-phase sinusoidal currents, the two pulse widths may be close to or equal to each other. The bus current section to be sampled is very short in the vicinity of the timing at which the two pulse widths are close to or equal to each other. Thus, the maximum phase of the voltage and the intermediate phase come close to each other, or the minimum phase and the intermediate phase of the voltage come close to each other, thereby disabling the current for two phases to be detected.
Japanese Unexamined Patent Application Publication No. 2012-65473 discloses a method of determining a size relation of the voltage levels of three phases corresponding to the pulse widths of three phases, rearranging the voltage levels of the pulse widths in a descending order of the pulse width, deciding a shift amount of the pulse that can detect a current for two phases, detecting a current from the shifted pulse waveform to decide a timing for AD conversion, and evaluating which phase the detected current is in from the pulse waveform. By doing so, a current for two phases is detected in such a way that the two PWM pulse widths do not come close to each other.